Projector for photographic transparencies

ABSTRACT

A slide projector wherein the slide changer is reciprocable between two end positions and through a median position in which a slide which is transported by the slide changer from the tray to projection position begins to open a diaphragm. The drive for the slide changer comprises a disengageable coupling which yields when the slide changer offers excessive resistance to movement in a direction to place a slide into projection position. A manually operable blocking device intercepts the slide changer before the latter reaches its median position so that the coupling is then disengaged and the slide which is pushed by the slide changer cannot reach its projection position and cannot open the diaphragm.

United States Patent PROJECTOR FOR PHOTOGRAPHIC TRANSPARENCIES 11Claims, 3 Drawing Figs.

US. Cl 353/103, 353/1 14, 353/122 Int. Cl G03b 23/02 Field of Search353/103,

[56] References Cited UNITED STATES PATENTS 2,949,814 8/1960 Boughton eta1 353/92 2,979,987 4/1961 Brumler et a1. 353/116 PrimaryExaminerLeonard Forman Assistant Examiner-Steven L. StephanAttorney-Michael S. Striker ABSTRACT: A slide projector wherein theslide changer is reciprocable between two end positions and through amedian position in which a slide which is transported by the slidechanger from the tray to projection position begins to open a diaphragm.The drive for the slide changer comprises a disengageable coupling whichyields when the slide changer offers excessive resistance to movement ina. direction to place a slide into projection position. A manuallyoperable blocking device intercepts the slide changer before the latterreaches its median position so that the coupling is then disengaged andthe slide which is pushed by the slide changer cannot reach itsprojection position and cannot open the diaphragm.

l FROJECTDR FOR PHOTOGRAHIIC TRANSPARENCIES CROSS-REFERENCE TO RELATEDAPPLICATION A somewhat similar slide projector is disclosed in thecopending application Ser. No. 799,558, filed on Feb. 17, i969 bylnlelmut Rube, and assigned to the same assignee.

BACKGROUND OF THE INVENTION The present invention relates to projectorsfor photographic transparencies, commonly known as slides, and moreparticularly to improvements in slide projectors wherein a diaphragmnormally prevents projection light from reaching the screen and is movedto open position only when or shortly before a slide assumes theprojection position in which light issuing from the projection lamp canpass therethrough and toward the screen.

It is already known to provide the slide changer of a slide projectorwith a motion transmitting member which can open the diaphragm in itsoperative position. The motion transmitting member is movable relativeto the slide changer between such operative and an inoperative position.When the operator decides to move or to maintain the motion transmittingmember in inoperative position, the slide changer transports a slide toand from projection position but the diaphragm remains closed so thatthe image of the slide is not projected onto the screen. A drawback ofsuch projectors is that the installation of the motion transmittingmember and of parts which can change the position of this memberinvolves considerable expense and also that the motion transmittingmember cannot be employed in all types of slide projectors, for example,in projectors where the diaphragm is opened by way of a slide while thelatter travels toward projection position.

It is also known to provide the drive which reciprocates the slidechanger in a slide projector with a disengageable or yieldable couplingwhich can transmit to the slide changer motion until and unless thelatter offers a predetermined resistance. Such couplings are employed toprevent damage to the slide changer and/or to slides when a slidehappens to be misaligned or when the slide changer meets an unexpectedobstruction which hinders its movement in a direction to move the slideto the projection position. The coupling is automatically reengaged assoon as the obstruction is removed or the position of a misaligned slidecorrected.

SUMMARY OF THEINVENTION An object of the invention is to provide a slideprojector of the type which embodies the aforementioned coupling with anovel and improved blocking device which enables the operator to skipone or more slides if the projection of images of such slides is notdesired.

Another object of the invention is to provide a blocking device whichcan be installed in a slide projector of the just outlined characterwith minimal expenditures and which occupies little room in and/or onthe housing of the slide projector.

A further object of the invention is to provide a blocking device whichcan be renderedineffective in a fully automatic way as soon as the drivefor the slide changer is set in motion upon completion of the cyclewhich would have resulted in projection of the skipped slide.

An additional object of the invention is to provide a slide projectorwherein the aforementioned blocking device cooperates with the couplingin a novel and improved way to enable the operator to skip one or moreslides without necessitating removal of such slide or slides from thetray and without influencing the mechanism which transports the tray toplace successive slides into the range of the slide changer.

The improved slide projector comprises a housing which accommodates aslidechanger movable between two end positions and through a medianposition to normally transport successive slides to projection positionduring successive travels from one to the other position, drive meansfor moving the slide changer andincluding a yieldable coupling whoseparts can be mounted on the slide changer and which normally drives theslide changer during movement from the one to the other end position,diaphragm means movable from a normal sealing position in which itprevents light emitted by the projection lamp from reaching the screen,motion transmitting means (preferably consisting of or including thatslide which travels from the tray toward projection position) operativeto move the diaphragm means away from sealing position in response tomovement of the slide changer from the median to the other end position,and blocking means including a blocking member movable by hand to anoperative position in which it intercepts the slide changer while thelatter receives motion from the coupling and before the slide changerreaches its median position so that the coupling yields and thediaphragm remains in sealing position.

The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theimproved slide projector itself, however, both as to its constructionand its mode of operation, together with additional features andadvantages thereof, will be best understood upon perusal of thefollowing detailed description of certain specific embodiments withreference to the accom panying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a transverse verticalsectional view of a slide projector which embodies the invention;

FIG. 2 is an enlarged view of a detail in the projector of FIG. I,showing the blocking device for the slide changer in operative position;and

FIG. 3 illustrates the structure of FIG. 2 but showing the slide changerin engagement with the blocking member.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. I, thereis shown a projector which comprises a housing 2 having legs I andprovided with a track 3 for a magazine or tray 4 which contains a seriesof slides 6. The tunnel which is formed in the housing for the track 3is open at its forward and rear ends and the tray 4 is normallyintroduced from the rear and is thereupon advanced stepwise so as toplace successive slides 6 into a first or transfer position occupied bythe slide shown in the left-hand position of FIG. 1. The tray 4 includescustomary transverse partitions 5 which define narrow compartments forthe slides 6.

A slide changer 7 serves to transport slides 6 from the transferposition to a projection position in which the trans parent portion ofthe slide extends across the path of light issuing from the projectionlamp so that the image is projected onto a screen, not shown. The slidechanger 7 is reciprocable along a horizontal guide rod 8 which ismounted in the sidewalls of the housing 2 and extends at right angles tothe longitudinal direction of the track 3. The slide changer 7 comprisesa first arm 9 which can move slides 6 from the transfer position toprojection position and a second arm l0 which can return slides fromprojection position into the corresponding compartments of the tray 3.

The track 3 is formed with customary ramps ill, I2 which respectivelyextend into complementary longitudinal recesses or cutouts I3, 14 in thebottom wall of the tray 5. The recesses l3, l4 communicate with thecompartments between the partitions 5, and the ramps Ill, 12 slopegradually forwardly and upwardly and thereupon downwardly so thatsuccessive slides 6 are lifted to a level above a sidewall llS of thetray 4 whereby the slide which reaches the illustrated transfer positioncan be withdrawn from its compartment by the arrn when the slide changer7 moves in a direction to the right and away from the end position shownin FIG. l.

The housing 2 accommodates two parallel horizontal guide rails l6, 16which are respectively located above and'below the projection gate andare provided with suitably contigurated channels for the upper and lowermarginal portions of frames of the slides 6. The left-hand ends of therails 16, i6 can receive the leading portion of a slide which is beingremoved from its compartment by the arm 9 of the slide changer 7. Suchslide thereupon moves along the channels of the rails to, 16' andultimately reaches the projection position in front of the projectionlamp, not shown. The surfaces surrounding the channels in the rails 16,16' are preferably roofshaped.

The drive means for reciprocating the slide changer 7 along the rod 8includes a shaft lid which is intermittently driven by an electric motor(not shown) or another suitable prime mover, a crank arm 17 which isaffixed to the shaft 18, and a crank pin ilt-provided on the arm 17 andarranged to orbit about the axis of the shaft 18. The motor which drivesthe shaft 13 is preferably installed in the interior of the'housing 2and the crank pin 19 cooperates with two different parts, namely, withan edge face 20 of the slide changer 7 which is engaged by the pin 19when the slide changer transports a siide from projection position backinto the corresponding compartment of the tray 4 and a flap 21 whichforms part of a yieldable or disengageable coupling and is engaged bythe pin H3 when the slide changer is to transport a slide from the tray4 toward and into the projection position. The shaft 18 rotates in aclockwise direction as indicated by the arrow. Of course, a fresh slide6 is moved to projection position during the interval when the crank pin19 moves away from engagement with the edge face 20 and towardengagement with the flap 21. The latter is pivotably secured to theslide changer 7 by a vertical pintle 22a which forms part of a hinge 22.A helical spring 23 has one of its ends attached to a post 24 on theslide changer 7; its other end is coupled to the flap 21 so that thelatter tends to assume the illustrated operative position in which itextends into the path of orbital movement of the crank pin 19. When theresistance which the slide changer 7 offers to movement with the crankpin exceeds a predetermined value, the spring 23 yields and permits theflap 2ll to pivot on the pintle 22a so that the pin i9 continues itsorbital movement but the slide changer 7 remains in a median position.The flap 21 constitutes a leaf of the hinge 22 and this hinge, togetherwith the spring 23, constitutes a disengageable coupling which formspart of the drive means and can transmit to the slide changer 7 movementuntil and unless the latter offers a predetermined resistance to suchmovement. A suitable stop, not shown, is provided on the slide changer 7to arrest the flap 2! in the illustrated operative position when thespring 23 is free to contract.

The slide projector further comprises a pivotable diaphragm or mask 26which normally overlies the projection gate to prevent illumination ofthe screen when the projection gate is empty, i.e., when there is noslide in the projection position. This diaphragm is a plate which isfulcrumed in the housing 2, as at 25, and is biased to sealing positionby a helical spring 27 which is coupled to a post 28 in the housing 2and a second post 29 of the diaphragm 26. The latter overlies theprojection gate during travel of slides between projection and transferpositions and opens shortly before a slide reaches the projectionposition. The means for pivoting the diaphragm 26 to open positioncomprises a two-armed motion transmitting lever 31 which is turnable ona pivot pin 30 of the housing 2 and the upper arm of which carries amotion receiving pin 32 extending into the path of movement of a slide 6which is being advanced toward projection position. The lower arm of thelever 31 carries a second pin 33 which abuts against an extension 34 atthe lower end of the diaphragm 26. When a slide advances toward itsprojection position, the leading edge of its frame engages the pin 32shortly before the slide reaches the projection position whereby thelever 31 turns in a clockwise direction and its pin 33 turns thediaphragm 26 in the same direction so that the diaphragm permits lightemitted by the projection lamp to pass through the transparent portionof the slide which reaches the projection position. The spring 27 storesenergy during movement of the diaphragm 26 from sealing to openposition.

The projector further comprises a novel blocking device 35 which can bemanipulated by hand and serves to prevent the transport of selectedslides 6 from transfer position to projection position. Thus, theblocking device 35 can insure that the diaphragm 26 remains in sealingposition during afull revolution of the crank pin 19 when the tray 4maintains in the transfer position that slide which the operator of theprojector does not wish to show to the audience. This is achieved byconstructing theblocking device 35 in such a way that it can hold theslide changer 7 in a median position so that the flap 2i yields whenengaged by the crank pin 19 and the latter merely turns withouteffecting any further rightward displace ment of the slide changer toits second end position. The arrangement is preferably such that theslide changer 7 can leave the illustrated end position but that itcannot advance a slide 6 so close to its projection position that theframe of such slide would cause the diaphragm 26 to leave its sealingposition.

The blocking device 35 is best shown in FIGS. 2 and 3. It comprises adepressible blocking member or stud 36 which is guided in a sleeve 37provided therefor in the top wall of the housing 2. The stud 36 has amanually depressible head 38 which is accessible from the outside of theprojector and can move at right angles to the guide rod 8. A helicalreturn spring 39 surrounds the upper part of the stud 36 and tends tomaintain it in the raised or extended position shown in FIG. 1. The stud36 cannot rotate in the sleeve 37 and its lower end portion is L-shapedto form a hook 36a. When in the extended position of HO. 1. the hook 36aof the stud 36 abuts against the bottom surface 37a of the sleeve 37.The notch 40 above the hook 36a accommodates a leaf spring d! whichconstitutes the yieldable element of a detent device further includingthe sleeve 37 and serving to releasably hold the stud as in depressed oroperative position against the opposition of the return spring 39. Theleaf spring 41 has a bent over free end portion or tongue 43a whichmoves to the left, as viewed in H6. 3, when the stud 36 is depressed tothereby engage the bottom surface 37a of the sleeve 37 and to thus holdthe stud 36 in operative position. An important advantage of the leafspring M is that the operator need not continue to press the head 38,i.e., it suffices to merely depress and to thereupon release the head 38as soon as the tongue Ma moves to a level below the bottom surface 37a.

The blocking device 35 further comprises a two-armed release lever 43which is turnable on a horizontal pivot pin 42 of the slide changer 7and is biased by a torsion spring 443 so that its left-hand arm normallyabuts against a stop 45 on the slide changer (see FIG. 2). Theright-hand arm of the lever 43 is provided with a bent over projection46 which can enter the notch 40 and can disengage the tongue Ma from thesleeve 37 so that the return spring 39 is then free to contract and tomove the stud 36 to a somewhat different operative position shown inFIG. 3. The upper arm of the lever 43 has a vertical shoulder 43a whichcan engage the adjoining vertical edge portion of the hook 36a when thestud 36 assumes the operative position of HO. 3. The stud 36 is free toreassume its extended or inoperative position shown in FIG. 1 when thecrank pin 19 engages the'edge face 20 and moves the slide changer 7 tothe left, as viewed in FIG 3, so as to withdraw the projection 46 fromthe notch 40 and to thus permit complete expansion of the return spring39.

THE OPERATION When the projector is idle, the crank pin i9 assumes theangular position shown in FIG. 1 in which the slide changer 7 dwells inits left-hand end position and is ready to transport a slide 6 fromtransfer position to projection position. if the operator thereuponstarts the motor which rotates the shaft 13, the crank pin 19 engagesthe flap 21 and moves the slide changer 7 in a direction to the right sothat the arm 9 engages the frame of the slide 6 and transports suchslide into the channeis of the guide rails i6, 16' whereby the slideultimately reaches its projection position. Shortly prior to reachingsuch position, the slide engages the pin 32 of the lever 31 and causesthe latter to pivot the diaphragm 26 in a clockwise direction so thatthe diaphragm permits observation of image on the slide which is locatedin the projection position. It is clear that, instead of pivoting thediaphragm 26 by way of the slide 6 and lever 3i, the projector may beequipped with diffcrent motion transmitting means mounted on the slidechanger 7 or on a part which moves with the slide changer, such motiontransmitting means serving to move the diaphragm to open position onlywhen the slide changer 7 is permitted to move all the way to the left,i.e., when the coupling including the flap 21 is not disengaged topermit the slide changer to dwell in a median position without actuallyplacing a slide into projection position. The ramps ll, 12 maintain theslide 6 which assumes the transfer position at a level above thesidewall 15 of the tray 4 so that such slide can readily travel abovethe wall 15 and is aligned with the rails 16, 16.

The crank pin 19 moves beyond the flap 21 as soon as a slide 26 reachesthe projection position. The motor which drives the shaft 18 isthereupon arrested, preferably automatically, for example, by opening aswitch which is installed in the motor circuit and is located in thepath of movement of the pin 19 and/or arm 17.

If the projector is not equipped with an automatic timer, the image ofthe slide in projection position is projected onto the screen as long asthe motor remains idle. Such slide is moved away from projectionposition in response to manual completion of the motor circuit wherebythe pin 19 engages the edge face 26 and causes the slide changer 7 toleave the right-hand end position and to move back toward the endposition shown in FIG. 1. The arm 10 of the slide changer pushes theslide from projection position back into the corresponding compartmentof the tray 4. The latter is thereupon advanced by a step in thecustomary way so as to locate the next-following slide in transferposition. All this takes place while the pin 19 moves away fromengagement with the edge face and toward engagement with the flap 21. Ofcourse, as the arm 10 pushes a slide from projection position, the frameof such slide permits the spring 27 to contract and to return thediaphragm 26 to sealing position shortly after the slide changer 7begins to move in response to engagement of the pin 19 with the edgeface 26. This insures that the screen remains unilluminated duringtransport of slides between the tray 4 and projection position.

As stated above, the tray 4 is advanced by a step after the justreturned slide reenters its compartment and while the crank pin i9travels from the edge face 20 toward the flap 21. As a rule, themechanism for advancing the tray 4 comprises a rack at the underside ofthe tray and a pinion which meshes with the rack and is indexed by theslide changer 7 or by a part which moves with the slide changer duringthe last stage of movement of the am 9 toward the position shown inFIG. 1. If the slide which has been moved to transfer position is one ofthose which should not be shown to the audience, the operator depressesthe head 36 of the stud 36 of the blocking device 35 during returnmovement of the preceding slide 6 toward transfer position, i.e., backintdthe corresponding compartment of the tray 4. The head 38 should bedepressed to such an extent that the tongue 41a of the detent spring 41moves to a level below the bottom surface 37a of the sleeve 37. Thespring 41 then expands and the tongue 41a engages the sleeve 37 (seeFIG. 2) to maintain the stud 36 in the operative position. As the slidechanger 7 travels toward the end position shown in Fit]. 1, the upperarm of the release lever 43 engages the hook 36a and the torsion spring44 yields to thus permit the lever 43 to bypass the stud 36. The springM causes the lower arm of the lever 33 to return into abutment with thestop 4-5 as soon as the projection 46 of the lever 43 moves beyond thestud 36. Such bypassing of the stud 36 by the lever 43 does notterminate the arresting action of the tongue 41a, i.e., the latterremains in engagement with the bottom surface 37a of the sleeve 37.

When the slide changer 7 thereupon begins to leave the end positionshown in FIG. 1 and its arm 9 pushes the slide which should not beexhibited away from transfer position, the pro jection 46 of the lever43 enters the notch 40 of the stud 36 and deforms the spring 41 so thatthe tongue 41a moves to the right, as viewed in FIG. 2, and the spring39 is free to expand slightly so as to withdraw the tongue 41a into thebore of the sleeve 37. However, the stud 36 cannot reassume its extendedposition because the projection 46 engages the hook 36a in a manner asshown in FIG. 3. In this second operative position, the stud 36maintains the left-hand edge face of its hook 36a in engagement with theshoulder 43a of the lever 43 so that the slide changer 7 is positivelyheld against further movement away from the tray 4. The slide changer 7is intercepted by the stud 36 before the slide 6 which is engaged by itsarm 9 can reach the pin 32 of the lever 31 so that the diaphragm 26remains in sealing position. Since the motor continues to drive thecrank pin 19 while the latter bears against the flap 21, the flap yieldsby stressing the the spring 23 so that the pin 19 continues its orbitalmovement without completing the movement of the slide from transferposition to projection position. The motor is then arrested in theaforedescribed manner and, when the operator thereupon again completesthe motor circuit, the crank pin 19 engages the edge face 20 and beginsto move the slide changer 7 from the median position shown in FIG. 3back toward the left-hand end position shown in FIG. 1. This causes thelever 43 to withdraw its projection 46 from the notch 40 of the stud 36whereby the return spring 39 is free to expand and to move the stud andits head 38 to the fully extended position of FIG. ll. Of course, if theoperator wishes to conceal the next-following slide too, the head 38 isdepressed during movement of the slide changer 7 back to the endposition of FIG. 1 so that the projection 46 reengages the spring 41during the next-following movement of the slide changer away from thetray 4. In this way, the operator can conceal as many slides as he orshe wishes. If only one slide is to be skipped, the head 38 of the stud36 is left untouched after the first depression whereby the blockingdevice 35 becomes inoperative in automatic response to return move mentof the slide changer 7 from the median position of HO. 3 to the endposition shown in FIG. 1.

An important advantage of the improved blocking device 35 is itsrelative compactness and simplicity. This is attributed to the fact thatthis blocking device cooperates with the coupling 21-24 of the drive forthe slide changer 7, i.e., with a component which is normally providedin the slide projector irrespective of the presence or absence of meanswhich enables the operator to skip one or more slides. Also, the noveland improved manner in which the blocking device 35 cooperates with thecoupling 21-24l is not affected by the fact that the diaphragm 26 can bemoved from sealing position by the slides 6 or by any other part orparts which share the movements of or move in synchronism with the slidechanger 7 while the latter travels from the median position toward theright-hand end position, as viewed in FIG. li, i.e., from the positionin which the diaphragm begins to open to the position in which thediaphragm is open and a slide normally occupies the projection position.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featureswhich fairly constitute essential characteristics of the generic andspecific aspects of my contribution to the art.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended.

lclaim:

1. In a slide projector, a combination comprising a slide changermovable between two end positions and through a median position; drivemeans for moving said slide changer and including a yieldable couplingwhich normally drives the slide changer during movement from one to theother endposition but can yield if said slide changer is blocked;diaphragm means movable from a normal sealing position in which itprevents projection light from reaching the screen to an open position;motion transmitting means operative to move said diaphragm means fromsaid sealing position to said open position during movement of saidslide changer from said median to said other end position; and blockingmeans including a blocking member movable by hand to an operativeposition in which the blocking member intercepts said slide changerwhile the latter receives motion from said coupling and before the slidechanger moves beyond said median position so that said coupling yieldsand the diaphragm means remains in its sealing position while thechanging cycle is completed.

2. A combination as defined in claim 1, wherein said blocking meansfurther comprises detent means for releasably holding said blockingmember in its operative position.

3. A combination as defined in claim 2, wherein said blocking meansfurther comprises release means for rendering said detent meansineffective in response to movement of said slide changer from said oneend position.

4. A combination as defined in claim 3, wherein said release meanscomprises a retaining member operative to engage said blocking member inresponse to movement of said slide changer from said one end position tothereby retain the blocking member in operative position.

5. A combination as defined in claim 4, wherein said blocking meansfurther comprises resilient means arranged to move said blocking memberaway from operative position when said detent means is inefiective andin response to movement of said slide changer to said one end position.

6. A combination as defined in claim 2, wherein said detent meanscomprises a sleeve reciprocably accommodating said blocking member and adetent spring provided on said blocking member and engaging said sleevein response to movement of the blocking member to its operativeposition.

7. A combination as defined in claim 6, wherein said blocking meansfurther comprises a return spring arranged to bias said blocking memberaway from said operative position.

8. A combination as defined in claim 7, wherein said blocking meansfurther comprises a release lever pivotally mounted on said slidechanger and arranged to engage said blocking member when it is in itsoperative position while said slide changer moves from said one endposition.

A combination as defined in claim 8, wherein said release levercomprises a projection and said blocking member is provided with a notchwhich receives said projection during movement of the slide changer fromsaid one end position when said blocking member is in its operativeposition.

It). A combination as defined in claim 9, wherein said projection isarranged to disengage said detent spring from said sleeve in response toentry into said notch so that said lever thereupon holds the blockingmember in operative position against the opposition of said returnspring.

11. A combination as defined in claim 8, wherein said release lever isyieldably mounted on said slide changer so that it can bypass saidblocking member during movement of the slide changer to said one endposition and while said blocking member dwells in its operativeposition.

1. In a slide projector, a combination comprising a slide changermovable between two end positions and through a median position; drivemeans for moving said slide changer and including a yieldable couplingwhich normally drives the slide changer during movement from one to theother end position but can yield if said slide changer is blocked;diaphragm means movable from a normal sealing position in which itprevents projection light from reaching the screen to an open position;motion transmitting means operative to move said diaphragm means fromsaid sealing position to said open position during movement of saidslide changer from said median to said other end position; and blockingmeans including a blocking member movable by hand to an operativeposition in which the blocking member intercepts said slide changerwhile the latter receives motion from said coupling and before the slidechanger moves beyond said median position so that said coupling yieldsand the diaphragm means remains in its sealing position while thechanging cycle is completed.
 2. A combination as defined in claim 1,wherein said blocking means further comprises detent means forreleasably holding said blocking member in its operative position.
 3. Acombination as defined in claim 2, wherein said blocking means furthercomprises release means for rendering said detent means ineffective inresponse to movement of said slide changer from said one end position.4. A combination as defined in claim 3, wherein said release meanscomprises a retaining member operative to engage said blocking member inresponse to movement of said slide changer from said one end position tothereby retain the blocking member in operative position.
 5. Acombination as defined in claim 4, wherein said blocking means furthercomprises resilient means arranged to move said blocking member awayfrom operative posItion when said detent means is ineffective and inresponse to movement of said slide changer to said one end position. 6.A combination as defined in claim 2, wherein said detent means comprisesa sleeve reciprocably accommodating said blocking member and a detentspring provided on said blocking member and engaging said sleeve inresponse to movement of the blocking member to its operative position.7. A combination as defined in claim 6, wherein said blocking meansfurther comprises a return spring arranged to bias said blocking memberaway from said operative position.
 8. A combination as defined in claim7, wherein said blocking means further comprises a release leverpivotally mounted on said slide changer and arranged to engage saidblocking member when it is in its operative position while said slidechanger moves from said one end position.
 9. A combination as defined inclaim 8, wherein said release lever comprises a projection and saidblocking member is provided with a notch which receives said projectionduring movement of the slide changer from said one end position whensaid blocking member is in its operative position.
 10. A combination asdefined in claim 9, wherein said projection is arranged to disengagesaid detent spring from said sleeve in response to entry into said notchso that said lever thereupon holds the blocking member in operativeposition against the opposition of said return spring.
 11. A combinationas defined in claim 8, wherein said release lever is yieldably mountedon said slide changer so that it can bypass said blocking member duringmovement of the slide changer to said one end position and while saidblocking member dwells in its operative position.